1. Field of the Invention
This invention relates to an optical apparatus of the kind having a lens control device for controlling the movement of a lens, such as a camera, a lens unit, a measuring equipment, a projection type TV set or the like.
2. Description of the Related Art
The volume and the weight of a lens part and an automatic focusing device arranged in a video-integrated type camera have recently come to decrease rapidly to permit a reduction in size and weight of the camera of this type. For this purpose, the automatic focusing device has been changing from an active type which has an infrared ray projecting and receiving means to a passive type which finds an in-focus point through a video signal coming from an image sensor without using the infrared ray projecting and receiving means. As for the lens part, a so-called inner focus type has come to be often used wherein a lens which serves to compensate for a shift of a focal plane caused by a power varying action is arranged to perform also a focus adjusting function so that a front lens located nearest to an object to be photographed can be fixed for reduction in
FIG. 4 shows the arrangement of the above-stated inner focus type lens. The illustration includes a first lens group 101 which is fixed; a second lens group 102 which is a zooming lens group performing a power varying action; a diaphragm 103; a third lens group 104 which is fixed; a fourth lens group 105 which is a focusing lens group performing the combined functions of compensating for a shift of a focal plane due to a power varying action and adjusting a focal point; and the image pickup plane 106 of an image sensor. The lens groups, the diaphragm 103 and the image pickup plane 106 are serially disposed one after another on an optical axis.
FIG. 5 shows changes taking place in position of the fourth lens group 105, in the arrangement shown in FIG. 4, for focus adjustment to each object distance in relation to the changes of focal length, i.e., changes in position of the second lens group 102. When there is no change in the focal length with the second lens group 102 in repose, the fourth lens group 105 makes focus adjustment by moving in parallel to the axis of ordinate for an applicable focal length, as shown in FIG. 5.
During the process of zooming, one of the loci of the fourth lens group 105 as shown in FIG. 5 is selected according to an applicable object distance, and driving control corresponding to the change of the focal length is performed on the fourth lens group 105 according to the locus thus selected. Under this control, an image can be obtained without any blur by adjusting the focus while compensating for any shift of focal plane resulting from power variation even during the process of zooming.
FIG. 6 shows by way of example a case where a zooming-lens moving area shown in FIG. 5 is divided into small areas. In this case, while the zooming-lens moving area is equally divided, the focusing lens positions are grouped together in such a manner that a plurality of loci of the focusing lens position considered to have about the same inclination are included within each of the divided areas of the zooming lens positions. In FIG. 6, arrows which vary their angles from moment to moment represent information on the speed of the fourth lens group 105.
In the case of FIG. 6, the moving range of the second lens group 102, on the axis of abscissa, is divided equally into a total of 16 areas, which hereinafter will be called "zoom zones". When the curves shown in FIG. 5 are divided by these zoom zones, these curves can be grouped into the portion having about the same inclination within each zoom zone. In a case where the speed of zooming is constant, the fourth lens group 105 can be allowed to move at a constant speed irrespectively of variations in the object distance as long as these curves have the same inclination.
Therefore, as shown in FIG. 6, the axis of ordinate is divided for every one of the zoom zones into parts in each of which the above-stated curves have the same inclination. Then, one representative speed is assigned to each of the divided parts. With the lens part arranged in this manner, the moving locus shown in FIG. 5 can be followed always at an optimum moving speed of the fourth lens group 105 by adjusting the lens part to an in-focus position at the beginning of zooming and then by performing the zooming action while detecting the positions of the zooming and focusing lenses.
In FIGS. 5 and 6, a term "WIDE (W)" means a wide-angle state, or a wide-angle end (position), where the zooming lens has the shortest focal length; and a term "TELE (T)" means a telephoto state, or a telephoto end (position), where the zooming lens has the longest focal length.
In the conventional lens control device, the inclination of the movement locus of the fourth lens group suddenly increases relative to the axis of abscissa in the neighborhood of the telephoto end, as shown in FIG. 5. This change indicates a sudden increase in the moving speed of the focusing lens in the neighborhood of the telephoto end. It has been known that this tendency becomes more salient accordingly as the focal length of the lens part increases.
It is, therefore, necessary to use such an actuator that gives a sufficient torque for realizing the speed desired in the neighborhood of the telephoto end. This necessity increases accordingly as the zooming magnification is increased. Generally, however, the actuator has a larger size, makes a larger noise and increases power consumption accordingly as its rotation speed and torque are increased. In other words, a greater zooming magnification necessitates an increase in the maximum drivable speed of a focusing-lens actuator accordingly. Then, it increases the size of the lens part, consumption of electric power and the size of a battery to be used for the camera. These things are detrimental to a reduction in size and weight of the camera.